Who is he and what would be the rationale behind appointing him (which important state/demographic could he help to win over?)
He's the former governor of Virginia and current Senator there. He's also a well known Clinton ally and would almost certainly land her VA, and if she wins, she wouldn't have to worry about the current governor (who is a Dem) appointing a GOP senator to replace Kaine, which would help the Dems potentially get the Senate back - which would be massive in terms of getting things done.